The present invention relates to tubes based on polyamides for transporting petrol and more particularly to tubes for conveying petrol from the tank of motor vehicles to the engine and to the hoses for transporting hydrocarbons in service stations.
For safety and environmental protection reasons, motor vehicle manufacturers require petrol tubes to have mechanical characteristics of strength and flexibility and enhanced permeability-resistance characteristics. The tubes must have the lowest possible permeability to petrol products and to their additives, particularly methanol and ethanol.
Polyamides have all these properties, however, in order to have good low-temperature mechanical properties, polyamides must be plasticized. But plasticized polyamides are less impermeable to hydrocarbons than unplasticized polyamides, especially with respect to lead-free petrols.
Patent EP 0,731,308 describes a tube based on polyamides for transporting petrol. This tube comprises an inner layer made of a polyamide/polyolefin blend having a polyamide matrix and an outer layer made of a polyamide. A binder layer and a layer of an ethylene-vinyl alcohol copolymer (EVOH) may be placed between the inner layer and the outer layer.
It is already known from Patent Application EP 0,781,799 that in motor vehicles, under the effect of the injection pump, the petrol flows at high speed in the pipes connecting the engine to the tank. In certain cases, the friction between the petrol and the internal wall of the tube can generate electrostatic charges, the accumulation of which may result in an electrical discharge (a spark) capable of igniting the petrol, with catastrophic consequences (an explosion). It is therefore necessary to limit the surface resistivity of the internal face of the tube to a value of generally less than 10.sup.6 ohms/square. It is known to lower the surface resistivity of polymeric resins or materials by incorporating conductive and/or semiconductive materials into them, such as carbon black, steel fibres, carbon fibres, and particles (fibres, platelets or spheres) metallized with gold, silver or nickel.
Among these materials, carbon black is more particularly used, for economic and processability reasons. Apart from its particular electrical conductivity properties, carbon black behaves as a filler such as, for example, talc, chalk or kaolin.